Process for preparing improved pigment materials



comprising titanium dioxide and Patented oer. 2s, 1941 UNITED s'ni'rias PATENT o'FFicE raocnss noa James E; Boole,

EJJllllollt-MN This invention relates to the production of nnely divided Piment I llterils. Hre D-ltiilularly -it relates to the production ofiwhite pig'- ment materials comprising wet ground' calcium carbonate. Still more' particularly it relates .to i

2,260,526 a marasma muevan mr nanas comvllmwilming- 1on,DeI.,aoorpcntionofDelawar-e stunning. Application November 4. im. I

semi No. mm

the production of white pigment materials j .wet ground calcium' carbonate of natural origin. j

The wet grinding of calcium carbonate mlnerals such as chalk, limestone, marble, etc., is known to those acquainted with the art of manufacturing Whiting. f It, is. however. not'nearly ascommon as dry grinding methods, doubtless 'I'hese devices are well suited of pigment materials particularly because of their ability to handle concentrated slurriesand eilect a size separation without the use of dispersing agents. They have\been used, for instance, to separate clay suspensions into a number of -size fractions by passing the suspension through several centrlfuges' in series at different r rates or by reclassifying the two iirst fractions on account of the added cost of drying the wet ground material. Whiting is useful in the'preparation of puttyl and as a illler for pawn-rubber. and the like, and also as an extender material for paints. Most commercial Whiting has been considered sumciently ilne if vabout 98% of. it .will pass a 325 mesh screen. This ilneness can be accomplished by dryigrinding but when a ilner product is desired it is usually necessary operating 'iiider varied conditions. In; these fractionating processes only a portion of the product is obtained which complies with' a given by successive passes through the same machine size range speciilcation. iConsequently, when a ilne product ls desired there is a coarse traction 'to be discarded. i

This invention has-as an object the production of tlnely divided pigment materials. A further object is the preparation of calcium carbonate pigment extender' material 'suitable for incoporation in gloss paints. A still further object is the preparation `of improved extenders by closed to employ wet grinding. Even the commercial wet ground products often show about 1 to 2% coarser than 325 mesh. This coarseness isap'- vparently permitted by the product s peclilcations rather than a limit oi' the wet grinding method since laboratory wet grinds have yielded products much lower in these coarse particles.

The use of commercial whitings for extenders in preparing paints is well The ground products find application 1in flat paints where'the surface oi' the coating `is not so sensitive to the coarse particles as a glossy film. A- good grade of Whiting having only 0.5% plus 325 mesh has` enough coarse particles to vimpart a rough or grittyfsurface to the paint nlm. To obtains tlrst class gloss paint essentially none of the particles should be greater than 6 microns in diameter. Calcium carbonates of particle size less than 6 microns can be made by precipitation methods and these products are known asipreclpitated whitings. But, while their grit characteristics do not interfere with film gloss they have oil absorption values too high for good extender ma- 90% of that for freshly fumed magnesium oxide.-

terial running 25 percent and higher. I'he most circuit wet grinding and centrifugal classification of naturally occurring calcium carbonate materials. A still further object is the preparation of calcium carbonate from the various natural sources such as limestone. chalk, marl,V calv cite, calspar. and other calcareous deposits in such a way that the product is substantially all in the crystalline form known as calcite. A still further object is the preparation of a composite pigment of excellent whiteness comprising a prime nigment and calcium carbonate. A still further object is the reduction Vin particle size of calcium carbonate. Additional objects will become ape parent from an examination of the following de.- scription and claims.

'I hese'obiects are attained according to the herein described invention which broadly comprises subiecting calcium carbonate having a power reflectance' in white light of not less than such as that produced in accordance with the process disclosed in the cgi-pending application of Ray L. McCleary, Serial No. 262,184. to thev steps of wet grinding in closed circuit with a continuous centrifugal classifier, blending with a prime pigment, dewatering, drying, and disintegratig.

' In a more restricted embodiment this invention comprises subjecting calcium carbonate raw material of lessthan about 4 mesh size and hav# ing a powder reectance in white light of not less 90% of that for freshly fumd magnesiumv ox`ide to the steps of het grinding in a closed cir-crut with a continuous centrifugal classnieril j for the preparation z blending with a prime pigment. dewatering, drying. anddisintegrating. y

A preferred embodiment of this-, invention comprises subjecting minus about iv mesh white calcium carbonate material, such as calcite,-

calspar, marble, chalk, etc., comprisingfat least 98% of calcium carbonate by weight and having a white light reflectance of at least 90% of that lfor freshly fumed magnesium oxide to the steps of wet ballmill grinding in a closed circuit with a continuous centrifugal classifier, blending with pigment grade titanium oxide, dewatering, drying, and disintegrating, said blending step being controlled to give 'a dried composite pigment comprising not more than 35% nor less'than 25% byweightof titanium dioxide, said calcium uct contained 29.25% by weight of T102 and was found to be an excellent pigment1 byl-aistudy Aof Color (whiteness) carbonate proportion of the product beingsubstantlally all in the crysta form of calcite.

In the specified steps of wet grinding in closed circuit with a continuous centrifuge, blending with a prime pigment, dewatering, drying` and disintegration, the blending step may be employed'either before the grinding step or following it. In the former case, the desired amounts vof theprime pigment, e.`g.,4 calcined TiOz and minus 4 mesh calcium carbonate raw extender material are fed to the. grinding mill along with enough water to facilitate good grinding; bothl components are thus ground and classified to- ,gethefrl Whenthe prime pigment is already available as a finely ground product the blending step may be carried out using the fine calcium carbonate suspension. discharged from the centrifuge.

This invention may be more readily understood from an examination of the following ex'- y amples which-are given for illustrative purposes and are not intended to place any restrictions or limitations on the-herein described invention.

EXAMPLE I Grinding followed by blending Calcite rock running about 99.3% calcium carbonate was crushed in a hammer mill to pass a 4` mesh screen. A 51/2' x 16 ball mill containing a normal charge `oi? flint-pebbles was charged as.y

ard freshly fumed magnesium oxide surface.

its properties as listed.

Oil absorption -A 14.5.

Tinting strength t20g When incorporated in a linseed oil paint and v applied the resulting film had excellent'gloss and was free of gritty particles.` i

I A Exmvrrtr- II Blending followed by grinding Minus 4 mesh calcite rock was prepared as in Example I and mived in a dry blending machine `with calcined4 titanium dioxide. The weight ratio of calcite rock'to titanium dioxide was 'l0/30'. The closed circuit grinding and classifying system used in Example I was employed again and when equilibrium conditions were obtained the ratio of calcite to titanium dioxide in the fine product was also 'l0/30. The fine solids were collected, filtered, dried, and disintegrated. Pigment tests showed:- Y

Oil absorption 15 Tinting strength 210y 'Color 16 This product was also very well suited for the Apreparation of gloss paints.

In order to give .a clearer understanding Aof the process and materials involved, the various tests employed will be briefiy described.

Reflectance.-This factor is usually measured relative to the reectance of light from a stand- Powder reflectance refers to reection from a with water and this -4 mesh calcite to give a V slurry concentration of 500 grams of solids per liter. The ball mill was'then operated until the solid particles were essentially reduced below 325 mesh. Continuous operation was then set up by Afeeding the ball mill discharge plus a'n additional equal amount of water (giving 250 grams solids per liter) to an 18" continuous centrifuge operating at 1200 R'. P. M. through a feed orifice 'y 3A inch in diameter. The coarse product delivered from the centrifuge was recirculated to the ballmill and the addition of water to the ball mill controlled to maintain a slurry concen-A tration of 500 grams solids per vvliter in the mill. The temperature of the slurry in. the mill was about 50 C; The fine solids were discharged from the centrifuge at the rate of 250 lbs. per hour so the feed rate of -4 mesh rock was also maintained at 250,1bs. per hour. Examination of the 'fines by elutriation showed 98.7 weight percent of the particles to be 6 microns or or less in dia eter. The slurry of nes was 'collected and blended with calcined ground titanium dioxide. After thorough mixing the composite 4suspension was.' filtered and dried. The dried pigment was then passed through a micropulverizer to' breakup the lumps'formed during the drying step. The, dried disintegrated prodcompact surface of the finely powdered material. The4 powdered sample may be compressed by a glass plate into a circular glass well 11A; inches by 1 mm. deep to give `a suitable surface. The

surface thus prepared is compared 4with the are limited to those showing a reflectance ofv at least ofA that for the standard MgO.

Oil absorption-,The weight in grams of linseed oil required to wet y grams of pigment is called its oil absorption. The method., of testing is described on pages 540-541 of the ,8th (Jan., 1937) edition of #Physical and Chemical Examination of Paints, vvarnishes, Lacquers, and

Colors," by Henry A. Gardner.

Colon- In white pigments the color is the apparent vwhiteness of an oil paste of a pigmentA relative to a standard sample similarly prepared and expressed in units on an arbitrary scale. The minimum perceptible difference. is regarded asone point -of color but a bluish cast is favored while yellowish tintsare penalized. The detalls of the procedure are described by J. E. Booge and Eastlack in Paints, Oil,' and ChemicalReview, April 9, 1924. A grading of 16 on the scale used represents excellent whiteness, while 12 is barely within the white pigment classification. v

Tinting strength-This test is designed to measure the ability of a pigment, in this case a white pigment. to cover or lighten the tint of a colored pigment when both are mixed'together in an oil paste. 'I'he test is describecLin Paint. ,Oil, and Chemical Review, April 16, 1924, by J.

E; Booge and H. E. Eastlack. The tintingstrength of a pigment is a good indication of its hiding power `in glossy oil paint illms. The best composite pigments comprising fine precipitated extenders and 30% of T10: l have tinting strengths of about 205 on the scaleused in evaluating the products of this invention.

It is to be understood that the specific embodiments .of this invention may be subjected to variation and modification without departing from the spirit and scope thereof. For example, a certain amount of variationin the operation of the closed circuit grind g system may be employed to modify the prod ct. Thus, the capacity of the mill may be inc eased while other variables are held constant in order to' increase the proportion of fine material and hence, increase the oil absorption of the product. It is sometimes desirable` to have an oil absorption as high as 20 for pigments used in gloss paints. In all cases, however,\the classification is made to keep the product of 6` micron size or nner.

While white calcite isl readily available and gives an excellent pro uct when processed according to this invention, it is to be understood that I may employ other forms of natural calf cium carbonate such as chalk, marl, marble, limestone, and the like. When forms other than calcite are employed, e. g.. amorphous -deposits of biological origin, or aragonite; I have found it desirable to maintain the temperature of the grinding slurry between about 30 C. and about An interesting feature of :aanmaningr the temperature of the grinding slurry above about 30 C. is the conversion of other forms of cal--l cium carbonate to calcite.

.t e most stable form of calcium carbonate even Apparently calcite is at very low temperatures. However, due to the various conditions of its formation in nature, it

may be in other crystal forms or amorphous as it is in some biological deposits and these unstable forms persist for years, in the massive state.

` However, the action of the grind which is a commicrons in diameter.

crons are returned for further grinding while, dueV 65 C., preferably between about 50 C. and about 55 C., in order to eilect the conv calcium carbonate to the stable f talline calcite. l

While I have found it desirable to employ titanium dioxide as my prime pigment it is to be understood that this 'thereto since it contemplates the employment of all prime pigments. e. g., zinc sulfide, calcium titanate, barium titanate, magnesium titanate, and

rsion of the rm of crysthe like. Although I am limited t; the use of a raw maa terial having a renectancf of 90% or better rela minerals to meet tive to MgO, I may improve the colo of some suitable methods such as crushing and wash-` ing, heating to destroyorg'anic matter, notation processes and various other means employed in the art of,ore dressing.

In preparing glossy paint films it is necessary to guard against the presence of pigment 'particles large enough to break the smooth mirrorlike surface of the iilm. I have found that particles greater than t6 micronsin diameter tend to protrude from the oil surface resulting in a ilat invention is not limited 511i this specification f-by various is made prior to its or gritty appearance. The pigment prepared according to thisy invention willnot interfere with the glossy character of the paint illm.v

Another necessary property of a true gloss paint pigment is rather low il absorption lying i below about 20. Fine precipitated extenders genl 'erally have a high oil absorption and due to this l oil demand, paints made from them do not have a highgloss dcsirablein enamel paints. This lack of gloss is not dueto the coarseness of the pigment but is 4due to the high oil absorption characteristic `whichleaves no excess of oil to fiow evenly over the surface of the paint nlm. The prerequisites for a gloss paint pigmentare,

' tween about 225 ysolids per liter after the calcite structure.

ally in the form'of bination of elevated temperature and pressure and classification a fortunate series of phenom-l ena occur. The' lumps of 'IiOz are rather loosely held'groups of very fine particles which disperse rapidly under the action of the ball mill toparticles most of which will be less than about 2 Meanwhile, the calcium carbonate granules are grinding at a slower rate but eventually all are classifier the calcite particles greater than 6 mi.

rangingdownward from 4.5 microns ,with-the bulk of the particles (due characteristics) lying in to the easy grinding paint pigment, the and, therefore, developing high hiding power while the extender component iscoarse enough to provide the correct oil absorption and yet no oversize or gritty material escapesdnto the product.

Attention is called to the fact that in'the examples contained herein dilution of the mill slurry passage through the centrifugal classifier, I have found that the optimum concentration for wet milling is considerably greater than the optimum concentration for passage through the centrifugal classifier. 'I'he examples specify 500 grams of solids per liter irj the ball mill and this should be regulated between the concentrations at 450 and 550 grams of solids per liter.V diluted 'to about fr this concentration or to bewhich it is'centrifugally classified, This is especially important in connection with the operation which "comprises the The the very ne range.. -This is exactly the results desired for/a gloss prime component being fine The mill discharge should be gramsfand about 275 grams of below 7 microns'. In the mixing or blending of titanium oxide calciner discharge with natural occurring calcium carbonate materials prior to the grinding and classification operation. Within these l*concentrations the two materials seem to have a grinding action upon each other that cannot-.be realized outside of this range. The titaniumvoxide as discharged from the calciner is made up of coarse lumps which are greater hiding power.

' mingle process out of two similar 'steps' in much 75 801mg per mer, nmfugauy gassin-mg to sepn porous due tothe sintering conditions prevailing. during the high temperature calcination ofthe product. The calcium carbonate material, on theother hand, is a mineral material fundamentally soft even though it is in coarser solid condition than the prime pigment material. In the grinding concentrations specified above the presence of the titanium dioxide seems to assist in the grinding of the calicum carbonate and the calcium carbonate' seems to assist in the grinding o f the prime white pigment which in this instance is titanium oxide; .1 Similar results-,are expected when other calcined prime pigments are substituted for titanium oxide.

The grinding ofthe two ingredients outlined above provides an easy meth'od of producing the low oil absorption, high hiding power extended calcium carbonate pigment which is so desirable for use in gloss paints. While the calcium carbonate material is being groundto lessthan 6 microns in size the titanium oxide isl being made finer and finer and this is not due to the grinding action of the balls in the mill alonefbut also t'o the grinding action of the particles of calcium carbonate material on each other and on the T102. In this manner the titanium oxide is ground to .the point where very few particles in excess of about l micron in diameter'exist and to the point wherev it exhibits maximumhiding power. At the same time a calcium carbonate material is: ground only to `the point where it is sufciently fine for use `in enamel films and yet remains in the low oily absorption range.A The passage of the ground Y slurry through the centrifugal classiiier then producesthe extended pigment having an oil absorption below about and substantially free of par;-

ticles having a size greater than 6 microns.

These properties' have been found to be most easily produced by following. the grinding conditions, outlined above. 4

The pigment prepared by this process if'lnds use in many of the pigment ields including paint, felt base coatings, -paper, rubber, etc. In the paint ield. particularly it is especially adapted to use in glass paints of high quality. Prepared as it is without the use of water soluble dispersing and fiocculating agents, -it is practically free of cemented agglomeraties which form when pigment materials are dried in the presence of sodium silicatefetc. Thispigment is readily wet with` most vehicles and consequently works intoL asmooth grit'free pain't with a minimum of mixing or grinding.

, The process is particularly useful in the cogrinding and classification of calcium carbonate as the prime pigment.' particularly TiOz, has to `coarse particles is effected with less grinding, a

situation which always tends to give a product of Agreater whiteness. Furthermore, a certain amun`t of control can be employed to modify the properties such as voil absorption while operating at the same preferred critical size separation of 6 microns.

-5. None of the commercial whitings listed in the U. S. Bureau of Mines Bulletin 395, match the characteristics of the product of this invention. All dry ground products are too coarse. All commercial wet ground products of satisfactory whiteness have about 40%' by weight in a particle size range above 6 microns. Some of these wet ground products have a satisfactoryoil absorption, i. e., about 20%, but when they are further ground as by laboratory wet ball milling the oil absorption becomes too great. All samples except three of dark colored chalks shown to have a particle size range essentially below 6 microns, have an oil absorption of 25% or greater.

As many apparently widely dierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specinc embodiment except as defined in the ap'- pended claims.

Having described the present invention the following is claimed as new and useful:

1'. A process for the production of finely divided calcium carbonate containing composite pigments having an oil absorption value of less than about 20 and substantially free of calcium carbonate particles larger than 6 microns which comprises subjecting calcium carbonate materlais having a white light reflectance of at least 90% in admixture with caloined titanium oxide to wet grinding at a slurry concentration of between about 450 to aboutl 550 grams of solids per liter, diluting the mill eiiluent to between about 225 and about 275 grams of solids per liter, centrifugally classifying to separate the fines and returning the coarse for further grinding.

2. 'A process for the production of iinely divid-u ed composite pigments having an oil absorption `value of less thanfabout 20 andl substantially free of calcium carbonate particles largertlian 6 microns which comprises subjecting calcium Y carbonate materials having a white light reilectand calcined titanium dioxide' as already pointed ance ofat least 90% in admixture with a calcined zinc sulde pigment to continuous wet grinding at a slurry concentration of between about 450 to about '550 grams of solids per liter, diluting the mill eilluent to between about 225 vand about 275 1. 'The use of the centrifuge permits the handling of concentrated' slurries without the useof dispersing. agents. Jonsequently, the space requirements and equipment size is considerably .less than processes using settling tanks.

v2. The elimination of dispersing agents reduces the cost of the product as well as yielding techv nical advantages already mentioned.

grams of ysolids per liter, centrifugally classifyingh to 'separate the fines and returning the-coarse for further grinding;

3. A processfor the production of' finely divided calcium carbonate containing composite pigments having an oil absorption value of less than 'about 20 and substantially free of calcium carbonate particles larger than 6 microns-which n'couipriaes subjecting natural calcium carbonate '3. The io-grinding and classmcauen or the f prime pigment with the extender is also advantageous. In the rst place, it provides an excell lent and thorough blending of the two components resultins in greater uniformity and possibly Secondly, it makes a materials having a white light reflectance ofv at' least in admixture with a calcined prime pigment to continuous wet grindingV at a slurry 'concentration of between about 450 to about 550 grams of sends per liter diluting aie mul emuent' tao-between about 225 Jand about 275 grams of mamans 5 arate the fines and returning the coarse for furthergrinding. A

4. A process for the production of finely dividd calcium carbonate containing composite Disments having an oil absorption value of less than about 20 and substantially tree of calcium car-v bonate particles larger than 6 microns `which comprises subjecting minus about 4 mesh white calcium carbonate material having a white light reectance of at least 90% in admixture with 10.

usines titanium dioxide to the steps or` wet beu mill grinding `in a closed circuit at aslurry con-v centration of between about 450 to about 550 grams oi' `solids per liter,l diluting the mill emuent to between about 225 and about 275 grams of solids per liter,'centrifugal1y classifying to separate the fines and returning the coarse for lurther grinding.

JAMES E. nooGE.

CERTIFICATE oF CORRECTION. recent Ne. 2,2605826. oeteber ze?, 19m.

.mms E. BooGE.

It iehereby certified that error appears in ,the printed specification ofthe above numbered patent requiringcorrection es follows: Page 1,seood column," line lil, for "'power" readpqwder; and page zysecond column, 1ine'vl5`, for mived read` mixed; and that the said Letters YPatent should beread with this correction therein tht the same my conion'n tothe` recoi. of the ease )11'1 the Patent'office.

signed 'end eeeledv this 9th dey ef December, A. D. 19;;1.-

i Henri van A redele, ($ea1) Acting Connnis sioner of Patents. 

